Is the monitor new?

And no one wants to pay a $100 assembly fee on a $300 computer (not counting your monitor) - there are much better deals than that.

For a new computer, with the monitor (assuming its actually worth $100 since I don't know much about monitors) I would say the computer would be worth $400 tops - and that's with the monitor. The difference between having you make it and purchasing a desktop from an actual company is an actual company has guaranteed support & warranty plus normally you get the OS for a very low price.